The present invention relates to a shared buffer memory type ATM communication system and method with a broadcast facility. More particularly the present invention relates to a communication system and method, such as an ATM switching system, ATM cross-connect equipment or subscriber's line demultiplex equipment, that form a communication network operating in the ATM (asynchronous transfer mode being) used for BISDN (broadband integrated service digital network) having a broadcast facility for sending information to a plurality of destinations.
In an ATM communication network used for BISDN, the information is transmitted in fixed length data blocks (data packets) called cells. The routing information is added to a head part of each cell, and in each transmission system/switching system, the transmission/exchange of the cell is performed in accordance with the routing information.
In BISDN, for example, for the distribution of video information, a video conference among multiple positions, and the like, a service for distributing the same information to a plurality of locations is provided. In such application, it is necessary to send the same cell to a plurality of destination systems. Therefore, in the switching system for example, "a broadcast processing" or "a broadcast facility" becomes essential which is used to produce a plurality of copy cells from one cell received from the calling system and to send the resultant copy cells to a plurality of destination systems in accordance with the routing information.
One example of a method of realizing a broadcast facility in an ATM switch is, for example, described in "Output Buffer Switch for Asynchronous Transfer Mode" by Hiroshi Suzuki et al., IEICE Technical Report, SSE88-172 (1988).
The ATM switch proposed in the above-mentioned article is designed in such a way that it is made up of a plurality of unit switches connected in multiple stages, and BCN (broadcast channel number) is added to an input cell which is to be subjected to the broadcast processing (hereinafter, referred to as "a broadcast cell", when applicable) by input interfaces which are connected to the subscriber's lines of the ATM switch and serve to supply the routing information to the broadcast cell.
Each unit switch includes a plurality of input ports, a plurality of output ports, and a plurality of output buffers provided in correspondence to the output ports, and operates in such a way that the input cells input from the input ports are routed to the associated output buffers, which are located in positions specified by the routing information which the individual cells have, and the cells stored in the output buffers are read out in sequence to the output ports. According to the above-mentioned prior art, in order to process the broadcast cell, each unit switch includes a broadcast control table for storing a bit pattern used to specify the output ports from which the broadcast cell is to be output, in correspondence to BCN of the broadcast cell. Thus, when the broadcast cell is input from one input port, a bit pattern for specifying the output ports is read out from that broadcast control table with BCN added to that broadcast cell as an address, and the copies of that broadcast cell are input to a plurality of output buffers specified by that bit pattern. The destinations of the broadcast cell are controlled by changing the contents of the broadcast control table.
The above-mentioned prior art ATM switch employs the structure which is called "an outgoing buffer system". However, in the ATM switch having such a type, the buffer having a fixed capacity is prepared every outgoing line. Therefore, in order to reduce the possibility of abandonment of the cell in the case where the cell to be output to the specific outgoing lines is input in a burst manner, it is necessary to make the memory capacity of each buffer large. As a result, there arises a problem that the system scale is increased.
If as in the prior art described above, the ATM switch of "the outgoing buffer system", employs the broadcast processing system for storing the copies of the broadcast cell in the output buffers corresponding to the destinations, a part of the precious storage capacity of each output buffer, becomes occupied. Since the capacity of the buffer memory is limited there is a fear that the capacity is insufficient even in the inputting of normal cells when taken alone without the occurrence of the burst. Therefore, there arises a problem that the capacity of the buffer is further increased.
On the other hand, an ATM switch is disclosed in U.S. Pat. No. 4,947,388 by H. Kuwahara et al. The ATM switch is designed in such a way that in order to effectively utilize the capacity of the buffer memory. The buffer memory is previously divided into a first area for storing the normal cell and a second area for storing the broadcast cell. The broadcast cell is temporarily stored in the second buffer area irrespective of the destinations, and the broadcast cell is read out from the second buffer area to a broadcast control circuit which duplicates the broadcast cell by the necessary number corresponding to the destinations to distribute the resultant copies to the associated outgoing lines.
As for the structure of the ATM switch itself, in addition thereto, there have been proposed other ATM switches having various kinds of types. For example, the structure called "the shared buffer type memory switch" which is shown in "A Memory Switch Architecture for ATM Switching Network" by Noboru Endoh et al, IEICE Technical Report, SSE88-56 (1988), has a type in which the buffer for storing temporarily the cell is shared among a plurality of outgoing lines. Thus, this structure has such an advantage that even if the so-called "burst" occurs in which a large number of cells to be output arrive at the specific outgoing lines in bulk, it is possible to reduce the possibility of the loss of the cells which is due to the insufficiency of the buffer capacity for each of those specific outgoing lines.
"The shared buffer memory type ATM switch is, in addition thereto, described in U.S. Pat. No. 4,910,731 by Y. Sakurai et al. for example.